1. Technical Field
This invention relates to error detection and correction and, more particularly, to an arrangement for detecting and concealing misdetected or miscorrected digital data such as may occur with digital video tape recorders and playback devices.
2. Description of the Prior Art
There are several digital videotape recorder and playback devices that can be found in the marketplace. It is known that, in such recorder/playback devices, a video digital signal, for example, a video digital signal having the well known 19 millimeter (mm), four times the frequency subcarrier (Fsc), also written as 4Fsc, digital composite format, also called the D-2 format, can be provided to a synchronization inner decoder. The inner decoder detects a synchronization pattern, for example, using the arrangement disclosed in U.S. Pat. No. 4,879,731 issued to R. K. Brush on Nov. 7, 1989 and entitled "Apparatus and Method for Sync Detection in Digital Data", and decodes and corrects the video digital signal using a Reed Solomon code arrangement, for example, using the arrangement disclosed in U.S. Pat. No. 4,597,083 issued to R. M. Stenerson on Jun. 24, 1986 and entitled "Error Detection and Correction in Digital Communication Systems". The inner decoder also provides an erasure flag and the decoded video digital signal to a frame memory and a segment memory.
The erasure flag can be a multivalued signal, for example, to indicate either (1) that no error was detected in the video digital signal and hence no error was corrected or that some maximum number of errors, for example, that one, two or three errors, were detected in the video digital signal and that the detected error or errors were corrected; or (2) that more than the maximum number of errors, for example, more than three errors, were detected in the video digital signal and that none of the detected errors was corrected. Accordingly, an erasure flag can be interpreted to flag or signal the event that, on the one hand, an error(s) was not detected in a video digital signal or that an error(s) was detected and was corrected; or that, on the other hand, an error(s) was detected and was not corrected in the video digital signal.
Commonly, the frame memory accepts video data, responsive to synchronization and identification information in the video digital signal, and thereupon buffers the video data so that the video data may be reformatted for outer decoding purposes. The frame memory also performs time base correction and provides facilities for freeze-frame, freeze-field, and other methods of operating upon the video digital signal. Also, signal conversion is typically done at an off-tape clocking rate. Thereupon, the video data is typically written into the frame memory and can later be read from the frame memory to the segment memory at the reference clocking rate, for example, at a rate of 2Fsc.
It is also known to extend the video data, which is typically read from the segment memory, to an outer decoder where outer code block columns of the video data are deshuffled and two or more video data channels are extended through an error correction code (ECC) outer decoding device. It is common that the decoding process corrects up to four flagged bytes per outer code block. After error correction, the video data and the corresponding erasure flags are written into a random access memory (RAM), sometimes called an intraline deshuffle memory. The writing into the RAM occurs in such a sequence that allows for subsequent intraline deshuffling of the video data. Thereafter, the deshuffled, corrected video data and concealment flags (viz-a-viz erasure flags) for indicating uncorrected data and, hence, for identifying video data that needs to be concealed are extended to a video error concealing device such as that disclosed in U.S. Pat. No. 4,792,953 issued to L. A. Pasdera and M. G. Lemoine on Dec. 20, 1988 and entitled "Digital Signal Error Concealment".
Unfortunately, the typical synchronization inner decoder, which includes an arrangement for detecting and correcting error(s), either (1) may not detect certain error(s) and hence does not correct those error(s), which can be called "unflagged errors", or (2) may detect certain error(s) but not correct those error(s), which can be called "flagged errors." It is also unfortunate that the typical outer decoder, which includes an arrangement for generally correcting flagged error(s) but not for correcting unflagged error(s), either (1) may not correct certain flagged error(s), which are then extended to a video error concealing device for concealment, or (2) may not correct unflagged error(s), which are then extended to the video error concealing device, but, unfortunately, are not concealed.